Beam advancement in magnetron beam switching tubes



Aug. 11, 1959 'E. SEIF BEAM ADVANCEMENT IN MAGNETRON pram swncamc TUBES 2 Sheets-Sheet 1 Filed Oct. 12. 1954 7 Aug. 11, 1959 2,899,551

. BEAM ADVANCEMENT IN MAGNETRON BEAM SWITCHING TUBES 7 Filed Oct. 12. 1954 2 Sheets-Sheet 2 INVEN ERIC S l TOR E F BY ATTORNEY BEAM ADVANCEMENT EN MAGNETRON BEAM SWITCHING TUBES Eric Seif, Philadelphia, Pa., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Application October 12, 1954, Serial No. 461,712

'10 Claims. (Cl. 25027) 'This invention relates to the magnetron type multiple position beam switching tubes, and particularly to means for advancing the electron beam in such tubes. Magnetron type multiple position beam switching tubes make use of crossed electrostatic and magnetic fields in their operation. Usually, the magnetic field is provided by a hollow cylindrical permanent magnet which surrounds the tube and whose flux permeates-the tube in lines which are-substantially parallel to an elongated centrally disposed cathode electrode within the tube. A typical tube of this general type has at least two arrays of electrodes surrounding the elongated cathode. An array ofsymmetrically disposed beam forming and directing electrodes, known as spade electrodes, surrounds the cathode and isconcentric with respect to it. Each spadeelectrodeis insulated from the other spadeelectrodes and in the operation of the tube isusually connected to a source of potential which is positive with respect to-the cathode through a spade load impedance circuit which includes a resistor. The spade electrodes are usually coextensive in length with the electron emissive portion 'of the cathode, and have a curved, usually U shaped, transverse cross sectional configuration. The open part of the spade faces outwardly with respect to the cathode. v

An array of symmetrically disposed electron receiving or target electrodes which has a larger diameter than the array of spade electrodes surrounds the spades and constitutes the outer array of electrodes of the tube. The target electrodes are generally equal in number to the spade electrodes, and eachtarget is aligned with the space between two adjacent spades whereby-electrons which pass through the space between thespades may impinge on the target electrode which is associated therewith. Like the spades, each target electrode is usually connected to a source of potential which is positive with respect to the cathode through an impedance member which includes a resistor. The output signal from each target electrode is developed across its target impedance member. An array of rod-like beam switching electrodes is positioned between the target and the spade arrays, there being one of the rod-like switching electrodes disposed between an edge of each'spade and a portion of an adjacent target electrode to which the beam is generally directed by means of the other adjacent spade. These switching electrodes provide reliable means for switching the electron beam from one stable beam position (spade and its associated target) to another stable beam position by means of negative pulses having an amplitude of only a few volts.

The operation of the above described typical tube is substantially as follows: When all of the spades are at the potential of the spade power supply, the relationship between the electrostatic field and the magnetic field is such that electrons emitted from the cathode follow curved paths around the cathode and substantially no electrons impinge on the spades or other outer electrodes of the tube. If, however, the potential on only one of the spades is lowered to, or near to, the potential of the States Patent "ice cathode by an external pulse, the configuration of the electrostatic field is changed, especially in the vicinity of the spade having the lowered potential, and a stream or beam of electrons is formed between the cathode and one edge of that spade to pass between two spades and impinge upon the associated target. The edge of the spade to which the beam is attracted is determined by the polarity of the magnetic field which permeates the tube. Because of the spade impedance member, the spade is maintained at reduced potential by beam current even after the external pulse expires and the electron beam locks in and remains at the edge of the spade which is farthest in the direction of rotation of the beam, and this edge is commonly called the leading edge of the spade. The electrons impinging on the edge of the spade cause electron flow through the spade impedance to reduce the potential of the spade sufliciently to maintain the beam locked in at that spade only if the spade impedance resistor value is properly chosen.

The beam switching electrodes may be biased positive, although they operate satisfactorily at some other reference potential such as ground or cathode potential. The electron beam is caused to switch from a stable position at one spade and lock in at another stable position at a further spade by applying a negative pulse to the switching electrodes. This negative pulse causes the beam to fan out or spread so that while the beam is locked in upon one spade some of the electrons impinge on the next advanced spade, thus causing some voltage drop through the spade resistor of the advanced spade.

. As the potential of the advanced spade drops, more and more electrons from the beam are attracted to and impinge on the advanced spade causing an additional voltage drop across the spade impedance. When the potential of the advanced spade reaches a sufiiciently low value, the electron beam will switch from the spade upon which it was locked in and lock in on the more advanced spade which now has the lower potential. The advanced spade is in the direction that the beam tends to rotate as determined by the polarity of the magnetic field.

The above method of advancing the beam is satisfactory in many cases, but has some disadvantages. For instance, if all switching electrodes are coupled together and the duration of a switching pulse applied to the switching electrodes is longer than the time required for the beam to switch from one position'to another, the beam will advance more than one beam position. Thus, in order to secure reliable switching by applying switching pulses to all the electrodes simultaneously the switching time required between each of the beam positions must be substantially the same and the duration of the switching pulse be carefully controlled to conform with this switching time. Since the beam may switch from one position to another in a tenth of a microsecond or less, it can be appreciated that some ditficulty will be encountered in making tubes on a mass production basis which will have the required close tolerances in switching time between the various beam positions.

Likewise, since the equipment with which these tubes may be utilized is not likely to have control pulses of the required duration, additional pulse shaping circuitry would be required to adapt the tubes for use with specific external circuitry.

The principal object of the present invention is to provide improved means for advancing the electron beam in a magnetron multiple position beam switching tube.

Another important object of the present invention is to provide improved, more reliable and simplified means for advancing the electron beam in a magnetron type multiple beam switching tube.

A further important object of the present invention is to provide improved means for advancing the electron beam in a magnetron type multiple position beam switching tube in a step-bystep manner.

The invention itself, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

Fig. 1 is an isometric view of a magnetron type multiple position beam switching tube which is suitable for use in connection with the present invention;

Fig. 2 is a sectional view, taken along the line 22 of Fig. 1;

Fig. 3 is a diagrammatical view of a magnetron type beam switching tube and electron beam advancing circuit in accordance with the present invention;

Fig. 4 is a simplified view of a coincidence detector circuit utilizing crystal diodes which may be utilized in controlling electron beam advancement in accordance with the present invention;

Fig. 5 is a simplified view of an electron tube coincidence detector circuit which is similar in function to the circuit shown in Fig. 4; and

Fig. 6 is an electron beam advancement control circuit in which the number of coincident circuits is less than the number of beam positions.

Referring to Figs. 1 and 2, there is shown a magnetron type beam switching tube 20 having an elongated indirectly heated thermionic cathode 22 which is centrally disposed within the tube envelope 24. Three arrays of electrodes surround the cathode. These arrays are usually concentric and coaxial with the cathode 22. The innermost array comprises a plurality in many cases) of elongated beam forming and directing electrodes 26 which are called spades. The spades have a somewhat U shaped cross sectional configuration, the base part 28 of the U facing towards the cathode 22. The spades are generally spaced symmetrically with respect to each other in the array.

The outer array comprises a plurality of elongated electron receiving output target electrodes 30, one of the target electrodes 30 being disposed adjacent to the space between each two adjoining spades 26. The target e1ectrodes 30 have a somewhat L shaped cross sectional configuration, and the base portion 32 of the L of each target extends into the space between the sides of one of the spade electrodes. The other portion of the target electrode extends across the space between two adjoining spades to receive the beam passing between the spades. A plurality of elongated rod-like switching electrodes 34, sometimes referred to as switching grids, constitutes an intermediate array of electrodes. A switching grid 34 is disposed between an edge of each spade electrode 26 and that side of a target 30 which extends between the two adjoining spades. A permanent magnet 36 provides a magnetic flux which permeates the tube 20 in lines which are substantially parallel with the longitudinal axis of the cathode 22.

Referring to Fig. 3, each of the spades 26 is connected via a separate load impedance member, illustrated as a resistor 38, to a source of potential such as the battery 40, which is positive with respect to the cathode 22. The cathode is shown for the sake of simplicity as being at ground potential. Each target 30 is connected to one source of potential 42 which is positive with respect to the cathode by a resistor 44, and to another source of negative potential 46 through two series resistors 48 and 50. An output signal may be taken from each target by way of one of the terminals 52 which is conductively connected to the junction between each resistor 44 and resistor 48. i The values of the resistors 44, 48 and 50 are chosen so that under static condition when the beam is not in the associated compartment the junction 68 between resistors 48 and 50 is at a potential equal to or 4 more positive than the switching grid biasing potential 54. Each of the switching grids 34 is connected to a biasing source 54 by way of an individual coincidence (and) circuit 56 and a common load resistor 60. Two specific forms of coincidence circuits which may be utilized are illustrated in Figs. 4 and 5, and will be described in detail later. It should be emphasized that the particular circuits of Figs. 4 and 5 are merely for the purpose of illustration, and that other coincidence detector circuits could be utilized in the practice of the present invention.

Electron beam advancing pulses are applied to the switching grid circuit through the input terminal 58 which is conductively connected to each of the coincidence circuits 56 and through a resistive impedance member 60 to the switching grid biasing source 54. Each target 30 is electrically coupled through a delay device 62, coupled from the junction between resistors 48 and 50 to the coincidence circuit 56 associated with the switching grid 34 which is closest adjacent to that target. In its simplest form the delay of device 62 may be obtained by connecting the junction between resistors 48 and 50 directly to the coincidence circuit 56 and establishing the necessary capacity to ground.

In operation of the circuit shown in Fig. 3, with the appropriate potential ditference applied between the cathode and spades and to the target electrodes, an electron beam will be formed between the cathode 22 and the beam position which includes the spade 26a, target 30a and the beam switching grid 34a if the spade 26a is grounded via the switch 64 or otherwise has its potential lowered to, or near to, the cathode potential. Once the electron beam is formed within the tube, it may be advanced by applying negative pulses to the input terminal It should be emphasized that the spade-cathode potential of the tube, as well as the switching grid bias potential, is determined by the electrode spacings and configurations and by the strength of the magnetic field which permeates the tube, and therefore cannot be specifically defined to cover the various operation ranges falling within the spirit of the present invention.

As previously mentioned, the prior art method of beam advancement in the tube 20 by the application of suitably polarized pulses to all of the switching grids simultaneously may result in spurious advancement of the beam unless the pulse width is closely controlled and unless the electrical characteristics at each beam position are substantially identical.

The possibility of such spurious advancement is avoided or greatly lessened in the electron beam advancement circuit of the present invention since a delay circuit 62 is provided between each target electrode 30 and a corresponding coincidence circuit 56 which is utilized to control the potential on the switching grid 34 which is associated with a partcular target. Thus, when the electron beam is advanced one beam position, the beam cannot be advanced further until the time delay of the target circuit expires. Thus, even though the pulse width or the switching characteristics of the individual beam positions are irregular, the delay circuit affords much greater tolerances in the switching circuit design.

Fig. 4 shows a coincidence circuit utilizing crystal diodes coupled between the target 30b and the switching grid 34b. The potentials of the batteries 42 and 46 together with the values of the resistors 44, 48 and 50 are such that under static conditions when the electron beam 66 is not locked in on the target 30b the potential at the junction 68 between the resistors 48 and 50 is approximately at ground potential. Likewise when no input switching pulse is applied to the pulse transformer 70, the secondary of that transformer is at ground or cathode potential. This potential is maintained also at the switching grid 34b by means of the clamping diode 76 which normally passes current from the potential source 77. Under these conditions the switching grid 34b is maintained at its static potential, which prevents the advancement of the electron beam to the next beam position.

It now the beam 66 impinges upon the target 30b, the potential at junction 68 will be reduced after such time as'necessary for the time delay capacitance 63 of time delay circuit 62 to' become charged. This potential charge is enough to cut off current through the diode 74, but current flowing through diode 72 will maintain the switching grid 34b at ground potential. An input negative switching pulse at transformer 70 acts in the same manner to permit current through diode 74 to maintain the switching grid 34b at ground potential when beam current is not impinging upon the target 30b. However, as both potentials are applied in unison, current flow from the battery 77 through the resistor 79 and diode 76 is blocked and the switching grid 34b is accordingly made negative so that the beam 66 is caused to advance to the next beam position. Coincidence circuits of this type are well known in the art and therefore may be readily adapted for use in the present invention. It is clear from this operation that successive input pulses from the switching source 80 will cause the beam to advance from one position to another. The input pulses may be converted by the transformer 70 and diode 72 into effective pulses having such duration that only one advancement of the beam occurs, should the input pulses otherwise tend to be of such long duration with respect to the delay circuit 62 that the beam might tend to advance more than one position.

In Fig. 5 an alternative coincident circuit utilizing tubes and known as the Rossi circuit 82 is afiorded. In the normal condition both tubes conduct sufficiently to maintain the grid 34b above ground potential. As two negative input signal pulses arrive at the respective tube grids, the conduction is reduced enough to cause the switching grid 34b to go negative because of battery 77, and thereby efiect switching of the beam.

Fig. 6 shows an embodiment of the present invention in which only two coincidence circuits are required to control the beam advancement for any even number of beam positions. Should the tube have an odd number of beam positions, three coincidence circuits would be required to control the beam advancement, the third coincidence circuit being required for the odd beam position. Each of the target electrodes 30' of the odd beam positions are coupled, through individual diodes 91 to a common lead 93 which is in turn coupled to the coincidence circuit A for controlling the beam advancement from all the odd beam positions by means of connection to a sub-set of alternate (odd) switching grids. Likewise, each of the targets 30" of the even beam positions are coupled through separate diodes 90 to the common lead 92 which is coupled to the coincidence circuit B for controlling the beam advancement from the even beam positions by means of a connection to a sub-set of alternate (even) switching electrodes. The odd beam position control may be eiiected by an unshown coincident circuit as the further sub-set of switching electrodes consisting of the single electrode at the odd beam position.

Although separate beam switching pulse transformers 70 are illustrated in Fig. 6, switching pulses could be applied to both the odd and the even coincidence circuits from a single pulse source if desired. The use of the diodes 90 and 91 in the target circuit between each target and the common leads 92 and 93 which are coupled to the separate coincidence circuits are for decoupling the targets from the respective coincidence circuits A and B when the electron beam is not locked in. With the diodes 90 and 91 in the target circuits, when the electron beam locks in at a beam position the target potential drops, making the diode conduct at that position to operate the coincident circuit in the manner described in connection with Fig. 4.

In the operation of the several embodiments described herein, when a negative pulse is presented to the grid associated'with the spade at-which the? beam is locked in, the beam will advance to the next position. This is accomplished without disturbing the eleetric'field of. the "beam tube, except in the compartment in which the beam resides when separate coincident circuits are used' for each switching electrode. Accordingly, the beam'formation characteris'tics within the tube is more reliably maintained during the switching operations.

With this invention it is also possible to count od'd numbers, as for Example 9, when-two adjacent tube targets of a decimal tube are coupled together, since it does not rely solely upon switching pulses inserted to alternately coupled groups of switching electrodes. Therefore, there is provided a reliable counter device capable of odd number counts.

Having therefore described the invention and its operation, those novel features believed descriptive of its nature and scope are defined with particularity in the appended claims.

What is claimed is:

1. An electronic system comprising a multi-position beam switching tube having beam forming structure, the tube having a plurality of beam receiving targets to accept the beam in any of its positions, the tube further having a set of switching means to cause the beam to advance from one target position to another, means deriving a first gating potential from a target electrode upon which the beam is impinging, means deriving a second gating potential from an external source, a separate gating circuit connected between each target and said switching means for causing the beam to switch from one target position to the next only in response to both gating potentials.

2. A system as defined in claim 1 wherein each target has a corresponding switching electrode, and each switching electrode is coupled to a separate gating circuit.

3. In a multi-position beam switching tube circuit wherein the tube has a plurality of beam receiving corn.- partments each containing a beam holding electrode for stably positioning the beam and a beam switching grid electrode for removing the beam from its stable position, a separate coincidence circuit coupled to each of said switching electrodes, means for coupling one switching signal commonly to all the coincident circuits from an external source.

4. A circuit as defined in claim 3 wherein the tube has a target electrode in each compartment, the signal gating potential is derived from one of the target electrodes, and normally non-conducting diode devices couple each of said target electrodes to one of said coincident circuits.

5. A circuit as defined in claim 4 wherein a separate coincidence circuit is coupled between each target electrode and the grid electrode associated therewith.

:6. An electronic system comprising a multi-position beam switching tube including a plurality of groups of electrodes, said groups comprising an electron beam forming electrode, an electron beam target electrode, and an electron beam switching electrode, first switching signal input means connected to each of said electron beam switching electrodes, a series connected delay circuit and coincidence circuit connected between each target electrode and its associated switching electrode, and second switching signal input means coupled to said coincidence circuit.

7. A multi-position beam switching tube circuit including a magnetron beam switching tube having a plurality of beam receiving compartments each containing a target electrode and a beam holding electrode for stably positioning the beam and a beam switching grid electrode for removing the beam from its stable position, said switching grid electrodes being connected in two sets with alternate ones of said electrodes being connected in one set, a separate coincidence circuit coupled to each set of said switching electrodes, and

means for coupling switching signals to said coincidence circuits.

8, The circuit defined in claim 7 wherein said target electrodes are each connected alternately to a separate one of said coincidence circuits.

9. The circuit defined in claim 7 wherein said target electrodes are each connected alternately through a diode to a separate one of said coincidence circuits.

10. The circuit defined in claim 7 wherein said target electrodes are each connected alternately to a separate one of said coincidence circuits with the target and switching electrodes of one compartment being coupled to the same coincidence circuit.

' References Cited in the file of this patent UNITED STATES PATENTS Haddad et al. May 1, 1951 Hough May 26, 1951 Skellett Dec. 2, 1952 Lair Sept. 8, 1953 Sternbeck Nov. 17, 1953 Lindberg et a1 Apr. 12, 1955 Ridler et a1. Aug. 7, 1956 FOREIGN PATENTS Australia Mar. 6, 1952 

